A Site-Conditions Map for California Based on Geology and Shear-Wave Velocity

by C. J. Wills, M. Petersen, W. A. Bryant, M. Reichle, G. J. Saucedo, S. Tan, G. Taylor, and J. Treiman

Abstract

One simple way of accounting for site conditions in calculating seismic hazards is to use the shear-wave velocity in the shallow subsurface to classify materials. The average shear-wave velocity to 30 m (Vs30) has been used to develop site categories that can be used for modifying a calculated ground motion to account for site conditions. We have prepared a site-category map of California by first classifying the geologic units shown on 1:250,000 scale geologic maps. Our classification of geologic units is based on Vs30 measured in 556 profiles and geological similarities between units for which we have Vs data and the vast majority of units for which we have no data. We then digitized the geologic boundaries from those maps that separated units with different site classifications.

Vs data for California shows that several widespread geologic units have ranges of Vs30 values that cross the boundaries between NEHRP-UBC site categories. The Franciscan Complex has Vs30 values across UBC categories B and C with a mean value near the boundary between those two categories. Older alluvium and late Tertiary bedrock have Vs30 values that range from about 300 to about 450 m/sec, across the boundary between categories C and D. To accommodate these units we have created intermediate categories, which we informally call BC and CD. Geologic units that have, or are interpreted to have, Vs30 values near the boundary of the UBC categories are placed in these intermediate units.

In testing our map against the available Vs30 measurements, we have found that 74% of the measured Vs30 values fall within the range assigned to the Vs category where they fall on the map. This ratio is quite good considering the inherent problems in plotting site-specific data on a regional map and the variability of physical properties in geologic units. We have also calculated the mean and distribution of Vs30 for each of our map units and prepared composite profiles, showing the variation of Vs in the upper 100 m from the available Vs data. These data show that the map categories that we have defined based on geologic units have different Vs properties that can be taken into account in calculating seismic hazards.

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